1. Field of the Invention
The present invention is directed to a method and apparatus for producing the perception of grayscale shading on digitally controlled displays. The invention is more specifically directed to a method and apparatus for generating the perception of many different brightness levels by digitally commanding the pixels of multiplexed liquid crystal displays towards either full brightness or complete darkness at appropriate times.
2. Description of the Prior Art
Grayscale shading can be generated on the screen of a conventional cathode ray tube (CRT) by varying an analog brightness control voltage at the grid electrode of the tube while an electron beam of the CRT is swept across different pixel positions of a display line. The same grayscale shading technique does not lend itself to digitally commanded displays such as multiplexed liquid crystal displays (LCD's), light emitting diode (LED) displays or plasma displays wherein individual pixels (discrete light source regions including emissive, transmissive and reflective types) can be commanded to switch towards only one of two brightness levels, ON or OFF (i.e., white or black). Such digital displays generally lack an analog control electrode equivalent to the grid electrode of a CRT and thus they do not have a direct means independent of their power lines for commanding a pixel towards an intermediate brightness level between black and white.
Multiplexed displays typically have only two electrodes provided at each pixel area for addressing a pixel area and energizing the pixel area to either produce the appearance of a fully lit (white) pixel or to produce the appearance of a fully darkened (black) pixel. Since an analog means for controlling brightness level is not available on many types of digital displays including raster-scan multiplexed displays, alternative digital techniques have been proposed for giving a viewer the perception of grayscale shading.
One of the proposed alternative techniques is a so-called "pulse-width modulation" scheme wherein the width of pixel energizing pulses is modulated between wide and narrow values to create a grayscale effect. Another technique is a so-called "frame-rate duty cycle modulation" scheme wherein the duration between energizing pulses of fixed width is varied between long and short to create a grayscale effect. A third proposed approach is to switch from the use of "time multiplexed" digital displays (which are commonly employed in laptop computers) to so-called "active matrix" display panels, the latter panels being ones which have active pixel-driving electronics defined at the location of each pixel, and to form a three-terminal analog-controllable display element out of such an active-matrix topology for continuously energizing each pixel discretely to a desired level of brightness.
While all the proposed grayscaling methods appear promising, each has its unique problems. Active matrix displays are still at an early stage of development. Techniques for increasing mass production yields and reducing manufacturing cost have not yet been refined. The idea of using an analog control scheme for creating the perception of grayscale shading discretely at each pixel does not conform with a general desire in the industry to have digital, time-multiplexed means rather than time-continuous analog circuits for controlling a display.
Conventional displays, such as raster-scan multiplexed LCD or electro-luminescent (EL) displays used in low-power laptop computers, are designed around the presumption that a pixel will be commanded to be either fully lit or unlit by a control pulse of a fixed duration (pulse width) which is associated with a binary logic bit (command bit). This presumption of a fixed-width command pulse conflicts not only with the basic requirements of the analog-controlled active matrix approach but also with the basic requirements of the pulse-width modulation approach. If the pulse-width modulation scheme is adopted, substantial redesign of already available display-driving electronics may be required to produce pixel energizing pulses of variable width and the cost of such new electronics may be significantly higher than that of the already-available drive electronics. Additionally, undesirable crosstalk between densely packed circuits may be created by employing the pulse-width modulation scheme since a high frequency harmonic content is typically associated with the relatively narrow pulses that might be produced by the latter technique. Because changes to the already proven technology of conventional panel-driving electronics are generally undesirable, the pulse-width modulation scheme is not presently acceptable within the industry.
The frame-rate duty cycle modulation method appears to be more acceptable to the industry at the current time, but this latter method is not without problems. Display elements based on light emitting diode (LED), liquid crystal (LCD), electro-luminescent (EL), plasma or other technologies typically require periodic application of refresh energy in order to retain their appearance of being lit or unlit (bright or dark). The frame-rate duty cycle modulation technique operates by extending the time between pixel energizing pulses. If the duration between energizing pulses is reduced substantially below a brightness integrating period associated with the human eye (i.e., substantially below one sixtieth of a second) in order to create a grayscale effect, a noticeable and undesirable perception of flickering within the display image may be created, particularly when a large number of pixels are being simultaneously commanded towards one of the ON and OFF states.
The use of alternate (interlaced) phasing has been proposed to overcome the flickering problem. The human eye has a tendency to integrate over space as well as over time. When adjacent pixels are operated in a grayscale mode by energizing them, for example, once every one thirtieth of a second instead of once every one sixtieth of a second as would be done for fully lit pixels, it is possible to suppress the perception of flickering by providing the lower frequency (30 Hz) waveform as two distinct signals, 180 degrees out of phase from each other, and applying the two signals respectively to adjacent pixels (adjacent in the vertical and horizontal directions) so that the overall refresh frequency for every pair of adjacent pixels still appears to be the higher 60 Hz rate rather than the more noticeable 30 Hz rate to the human eye.
Unfortunately, when the interlaced phasing method is attempted for producing more than three levels of brightness, without further refinement, other visual disturbances tend to appear. Long, continuous lines of pixels that are alternately turned on and off, approximately 120 or less degrees out of phase from adjacent lines tend to produce a so-called "movie marquee" effect wherein it appears that the surface of a grayscale shaded area is streaming either to the left, to the right, diagonally or in circles. This appearance of a streaming motion (movie marquee effect) can be distracting to the eye and is therefore undesirable. Image jitter and transitory spots arising from crosstalk, noise, etc can also create problems and give the viewer an undesirable perception of a fuzzy rather than crisp, focused image. Heretofore, an effective method for simultaneously avoiding multiple visual disturbances such as the flickering effect and the movie marquee effect has not been shown.